1. Field of the Invention
The present invention relates generally to transistors, and more specifically to a bipolar transistor structure for improved junction breakdown voltage and junction capacitance characteristics.
2. Description of the Prior Art
For designers of bipolar transistors which are expected to generate power gain, it is important that transistor design address both higher voltage and current requirements. Devices which generate high power gain, such as Microwave and RF power transistors, must possess higher voltage and current capabilities than those transistors which do not provide power amplification.
In order to achieve optimal transistor power gain, the correct balance between several electrical parameters must be found. The proper voltage and current values must be found. Increasing the resistivity increases the voltage level which may be achieved, but at the same time decreases the current value. Therefore, the correct balance between the lowest possible resistivity and the highest possible breakdown voltage must be achieved. In addition, other electrical characteristics such as base-collector capacitance must be minimized.
Under reverse bias conditions, the transistor's base-collector junction breakdown voltage value may be adversely affected. Defects and impurities on the silicon crystal surface where the base-collector junction meets the surface of the crystal conspire to lower the junction breakdown voltage. The high breakdown voltage characteristic of bulk silicon is not achievable. Because of the nature of diffused junctions, especially shallow junctions, typically about 40% of the bulk breakdown voltage can be obtained.
In order to enhance the junction breakdown voltage for a given resistivity level, physical structures such as depletion rings and mesas have been used. These structures successfully enhance breakdown voltages when used in conjunction with appropriate etching, washing, and heating techniques in suitable environments. The surface structure is often stabilized through passivation with oxide or nitride.
Physical structures work to minimize surface effects, but do possess undesirable characteristics. Depletion rings, single and multiple concentric, are beneficial in that higher junction breakdown voltages may be obtained. However, the addition of each depletion ring adds to junction capacitance, an undesirable parameter to RF and microwave performance. The larger the area attributed to depletion rings, the higher the junction capacitance, all other parameters being equal.
Mesa structures do not employ depletion rings; therefore, no significant junction capacitance is added as a result of using mesa structures. However, the base-collector junction still protrudes into the crystal silicon surface, causing only moderate junction breakdown voltages to be achieved.